We are interested in the molecular bases of the regulation of the actin-activated ATPase activities of class-I and class-II myosins, the biological roles of these myosins and the regulation of their biological activities. In one project, to understand the coupled contributions of myosin heads and tails to enzymatic activity and function, we studied the properties of chimeras consisting of the motor domain of Dictyostelium myosin II and the tail domains of either Acanthamoeba or smooth muscle myosin II. The chimeras have 10-15 higher actin-activated ATPase activity than wild-type Dictyostelium myosin II but, in contrast to wild-type myosins, this activity is not regulated by phosphorylation of either the regulatory light chain or, in the Acanthamoeba chimera, phosphorylation of the tail. When expressed in myosin II-null cells, both chimeras rescue two myosin II-dependent functions, cytokinesis in suspension culture and capping of con A receptors, but do not support a third myosin II-dependent function, full development to fruiting bodies. It had previously been shown by others that the tail domain of Dictyostelium myosin II contains all the information needed for localization to the cleavage furrow (CF) of dividing cells. We have now found that the 283-residue assembly domain, the shortest tail segment that assembles in vitro, goes to the CF when expressed in myosin II-null cells, as does also an equivalent 256-residue region from the tail of Acanthamoeba myosin II. Thus, there seems to be no specific sequence requirement for localization of myosin to the CF. We have compared the abilities of 15 mutants of the HCM/TEDS-site surface loop of Dictyostelium myosin II to support the three myosin-II dependent functions. The chicken smooth muscle myosin loop supports normal con A-receptor capping, 80% of normal growth and only slightly impaired development. The human beta-cardiac myosin loop supports growth at only 25% of wild-type rate, incomplete capping and greatly impaired development, i.e. to the the tipped mound stage, only slightly better than myosin II-null cells. Various point mutations make the smooth muscle loop fully functional and the cardiac loop function more like the smooth muscle loop. The actin-activated ATPase and in vitro motility activities of all of the mutants are being studied to see which, if either, correlates with their biological properties. Studies of chimeras in which the heavy chain head, neck and tail domains and light chains of Acanthamoeba and Aspergillus myosin I are interchanged in various combinations provide further evidence for the interdependence of the three heavy chain domains and light chains for myosin activities.